Nuclear medicine imaging is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (e.g., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
For example, in PET and SPECT nuclear medicine imaging systems, emitted gamma photons are typically detected by placing a scintillator over the region of interest. Such scintillators are conventionally made of crystalline material such as thallium activated sodium iodide, NaI(Tl) or leutetium oxyorthosilicate (LSO), which interacts with absorbed gamma photons to produce flashes of visible light. The light photons emitted from the scintillator crystal are in turn detected by photo sensor devices that are optically coupled to the scintillator crystal, such as photomultiplier tubes or avalanche photodiodes (APD). The photo sensor devices convert the received light photons into electrical pulses whose magnitude corresponds to the amount of light photons impinging on the photosensitive area of the photo sensor device.
Not all gamma interactions in a scintillator crystal can be used to construct an image of the target object. Some of the interactions may be caused by gamma photons that were scattered or changed in direction of travel from their original trajectory. Thus, one conventional method that has been used to test the validity of a scintillation event is to compare the total energy of the scintillation event against an energy “window” or range of expected energies for valid (e.g., unscattered) events. In order to obtain the total energy of the event, light pulse detection voltage signals generated from each photo sensor device as a result of a single gamma interaction are typically integrated from the start of each pulse, and then added together to form an energy signal associated with a particular event. Energy signals falling within the predetermined energy window are considered to correspond to valid events, while energy signals falling outside of the energy window are considered to correspond to scattered, or invalid, events, and the associated event is consequently not used in the construction of the radiation image, but is discarded. To get an accurate measure of the event from an output of an analog-to-digital converter (ADC), the value of the ADC zero (baseline) is typically subtracted from each sample used to form the integration sum of the scintillations. The baseline value in both PET and SPECT systems can be affected by DC offsets in the ADC and amplifiers, shifts to the AC coupling as a function of count rate, noise from the detection photo sensor and large noise voltages induced by a gradient coil in a magnetic resonance imaging (MRI) system.
Desirable in the art is an improved nuclear medicine imaging system that would determine a more accurate integrated value of scintillation events in PET and SPECT nuclear medicine imaging systems.